In the recovery of oil from oil-containing formations, it is usually possible to recover only minor portions of the original oil-in-place by so-called primary recovery methods which utilize only natural forces. To increase the recovery of oil a variety of supplementary recovery techniques are employed. These techniques include waterflooding, miscible flooding, and thermal recovery.
A problem that arises in various flooding processes is that different strata or zones in the reservoir often possess different permeabilities. Thus, displacing fluids enter high permeability or "thief" zones in preference to zones of lower permeability. Significant quantities of oil may be left in zones of lower permeability. To circumvent this difficulty the technique of profile control is applied to plug the high permeability zones with polymeric gels and thus divert the displacing fluid into the low permeability, oil rich zones. Among the polymers examined for improving waterflood conformance are metal cross-linked polysaccharides, metal cross-linked polyacrylamides, and organic cross-linked polyacrylamides.
Basic to the problem of diverting displacing fluid with polymeric gels is the necessity of placing the polymer where it is needed, i.e. selective penetration into the high permeability zone. This is possible in the case of Cr cross-linked xanthan gel formed above ground. Xanthan biopolymers may be cross-linked with metal ions such as Cr.sup.+3 above ground to give gels. These gels are shear stable and thinning and can be injected into the formation where they then reheal. Due to the unique rheological property of such a gel, it prefers to go into high permeability zones. However, Xanthan-Cr gels have poor thermal stability at temperatures greater than about 140.degree. F. There are also many other gels systems that are formed in-situ. One system disclosed in U.S. Pat. No. 3,557,562 contains acrylamide monomer, methylene-bis-acrylamide as an organic cross-linker, and a free radical initiator. This system undergoes polymerization in the formation to give a polyacrylamide cross-linked with methylene-bis-acrylamide. However, the viscosity of the solution when injected is like that of water. Unless mechanical isolation is used, these solutions are quite capable of penetrating low permeability, oil bearing zones. Another form of in-situ gelation involves the injection of polyacrylamide containing chromium in the form of chromate. A reducing agent such as thiourea or sodium thiosulfate is also injected to reduce the chromate in-situ to Cr.sup.+3, a species capable of cross-linking hydrolyzed polyacrylamide. Even though the polyacrylamide solution has a viscosity greater than water, it is not capable of showing the selectivity that a gel can. Thus, polyacrylamides cross-linked with chromium in-situ can also go into low permeability zones. It is not useful to cross-link polyacrylamides above ground and inject them as gels, because polyacrylamide gels undergo shear degradation.
Therefore, what is needed is a method where a selective ex-situ gel composition can be combined with an in-situ gel composition so as to obtain greater selectivity in closing a zone of greater permeability in a formation while forming a gel having substantially better qualities to withstand formation conditions.